Coherent light generators – Particular beam control device – Q-switch
Reexamination Certificate
2002-07-29
2004-08-31
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Q-switch
C372S018000, C372S019000, C372S098000, C372S100000
Reexamination Certificate
active
06785303
ABSTRACT:
BACKGROUND OF THE INVENTION.
The invention concerns a process for the generation of ultra-short laser light pulses, in particular a process for stabilizing of the operation of a pulse laser, and a process for the production of highly accurate optical frequencies, and a laser device for the generation of ultra-short light pulses, in particular a frequency stabilized pulse laser, and applications of such a laser device in spectroscopy, time or frequency measurement technology and communications technology.
The production of ultra-short laser light pulses (light pulses with a typical pulse duration in ns- to fs- range) known since the 1970s is based on so-called mode synchronization. In a laser medium many fundamental oscillations with different frequencies may be generated, given sufficient bandwidth of the laser transition in the resonator. If a suitable mechanism is provided to produce a stable phase relationship between the fundamental oscillations (mode synchronization), short light pulses with a time separation are produced, being equal to the quotient of the doubled resonator length and circulation velocity of the pulses, and corresponding in their spectroscopic composition to the optical frequencies generated in the resonator and contributing to the formation of the pulses.
A so-called frequency comb is obtained from a Fourier transformation of the course of intensity of the laser beam's pulse form, formed through &dgr;-type functions of optical frequencies contributing to each pulse and whose envelope is within the bandwidth of the laser transition in the laser medium. Essentially, the width of the envelope is inversely proportional to the pulse duration. An example for such a frequency comb is shown schematically in FIG.
5
. Each contributing frequency to such a frequency comb is designated as mode M. Corresponding to the longitudinal laser modes, the frequency separations of the elements of the frequency comb are even number multiples of the pulse repetition frequency f
r
=&tgr;
−1
(repetition rate) . The comb structure of fspulses in the frequency space is described, for example, in “Femtosecond Laser Pulses” (C. Rulliere, ed., Springer Publications, Berlin 1998).
Since the pulse repetition frequency f
r
depends on the resonator length, deviations of the ideally stable mode intervals occur even in the presence of most insignificant instabilities of the resonator. Techniques for the stabilization of the resonator length are known, which suppress changes in the mode intervals. To this end, a movable resonator end mirror in the direction along the length of the resonator is installed and adjusted in the presence of a mode shift through the use of a control loop. This conventional stabilization method is insufficient for the present-day accuracy requirements in the use of spectroscopy or time measurement technology.
J. N. Eckstein et al. (see “Physical Review Letters”, Vol. 40, 1978, p. 847 et seq.) recognized that the row of the modes would be suitable as a scale for frequency calibration At the same time the insufficient stability of the pulse laser and noise-related shifts in mode frequencies were pointed out. It was observed that the shifts continued to occur in spite of the stabilization of the resonator length. According to L. Xu et al. In “Optics Letters”, Vol. 21, 1996, p. 2008 et seq., the cause is the group velocity of a pulse, which determines the circulation time in the resonator and therefore the repetition frequency, as a rule does not correspond to the phase velocity of the individual modes. The modes separated by the even number multiples of the repetition frequency cannot be represented in their absolute frequency position through even number multiples (n) of the repetition frequency f
r
, but rather through the sum (n·f
r
+f
p
) of n·repetition frequency f
r
and a so-called phase slip frequency f
p
which has the same value for all modes, corresponding to the quotient of the respective phase differences from pulse to pulse through the circulation time (2Π)&tgr;. A determination of these phase differences is until now not available, so that the use of pulse lasers for measuring purposes or as generators of optical frequencies is limited.
In the following, two areas in which there is an interest in highly accurate frequencies will be described. The first application concerns frequency measurement generally, in particular making time or frequency standards available. The second application lies in the area of spectroscopy, in particular the measurement of atomic electronic energy transitions.
A widely used time standard is given through the so-called cesium atom clock with a base frequency of 9.2 GHz. The time measurement is carried out through a direct counting out of the base oscillations, which is presently possible with a degree of accuracy of for example 10
−4
. Significantly higher relative accuracy up to the magnitude of 10
−18
are expected from optical frequency standards, for example on the basis of cooled ions in field cages (see, e.g., M. Roberts et al. In “Physical Review Letters”, Vol. 78, 1997, pp. 1876 et seq.) or from the extremely narrow atomic resonances such as that of the 1S-2S-transition of hydrogen (see, e.g., T. Udem et al. In “Physical Review Letters,” Vol. 79, 1997, pp. 2646 et seq.). These frequency standards possess;, however, optical frequencies above 80 THz, which can no longer be counted out directly electronically. For an optical clock, one needs therefore an apparatus for frequency transformation from the high frequency of the frequency standard to a low frequency that can be evaluated through electronic means. Such an apparatus possesses the function of a “clockwork” for an “optical clock.”
Harmonic frequency chains are used for the bridging of the large frequency separation between optical frequencies and. (electronically countable) radio frequencies (see H. Schnatz et al. In “Physical Review Letters”, Vol. 76, 1996, P. 18 et seq.). With a harmonic frequency chain, a reference frequency is multiplied with even number factors at a number of multiplication stages, until the desired frequency is attained. This requires, however, a separate transfer oscillator with a phase coupling to the preceding harmonic signal for each multiplication level. The availability of a numerous oscillators at various frequencies makes the setup voluminous, complicated, and expensive.
FIG. 13
illustrates a further principle of a well-known scaling stage for optical frequencies (see T. W. Hänsch in “The Hydrogen Atom”, ed. G. F. Bassani et al., Springer Publishing, Berlin 1989, p. 93 et seq.; H. R. Telle et al. In “Optics Letter”, Vol. 15, 1990, p. 532 et seq.; and T. W. Hänsch in “Physikalische Blätter,” Vol. 54, 1998. P. 1007 et seq.). If one overlaps two laser beams that differ only slightly in frequency onto a photo detector, one observes a modulation of light intensity at the differential frequency (beat signal). This beat signal may be used to adjust the frequency of one partial beam onto the frequency of the other partial beam. According to the scheme of
FIG. 13
, two laser frequencies f
1
and f
2
are compared to a third laser frequency f
3
near the middle frequency (f
1
+f
2
/2). The summation frequency f
1
+f
2
is produced with a nonlinear crystal (+), and the upper harmonic vibration
2
f
3
is produced with a further nonlinear crystal (×2). The lower frequency beat signal at the photo detector is used in the digital control loop &PHgr; to control the frequency and phase of the third laser to oscillate precisely on the middle frequency, i.e. f
3
=(f
1
+f
2
)/2. Thus a frequency interval &Dgr;f is reducible by a factor ½
n
with a chain of n scaling stages according to FIG.
13
. If one begins such a chain of scaling stages with a laser frequency f and its second upper harmonic 2f, that is, with &Dgr;f=f, then one obtains a differential frequency of f/2
n
after n scaling stages. The problem with the described scaling chains is that at least tw
Hänsch Theodor W.
Holzwarth Ronald
Reichert Jörg
Udem Thomas
Al-Nazer Leith A
Duane Morris LLP
Ip Paul
Max-Planck-Gesellschaft zur Forderung der Wissenschaften e. V.
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